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An edge effect is the effect of the juxtaposition of contrasting environments on an ecosystem. This term is commonly used in conjunction with the boundary between natural habitats, especially forests, and disturbed or developed land. Edge effects are especially pronounced in small habitat fragments where they may extend throughout the patch.

When an edge is created to any natural ecosystem, and the area outside the boundary is a disturbed or unnatural system, the natural ecosystem is seriously affected for some distance in from the edge. In the case of a forest where the adjacent land has been cut, creating an openland/forest boundary, sunlight and wind penetrate to a much greater extent, drying out the interior of the forest close to the edge and encouraging rampant growth of opportunistic species at the edge. Air temperature, vapour pressure deficit, soil moisture, light intensity and levels of photosynthetically active radiation (PAR) all change at edges.

In a 1994 paper Skole and Tucker estimated that the amount of Amazonian area modified by edge effects exceeded the area that had been cleared. Forest fires are more common close to edges as a consequence of increased desiccation at edges and increased understory growth present due to increased light availability. Increased understory biomass provides fuel that allows pasture fires to spread into the forests. Increased fire frequency since the 1990s are among the edge effects which are slowly transforming Amazonian forests. The amount of forest edge is also orders of magnitude greater now in the United States than when the Europeans first began settling North America. Some species have benefitted from this fact, for example the Brown-headed Cowbird, which is a brood parasite that lays its eggs in the nests of songbirds nesting in forest near the forest boundary. Thus, the more edge in relation to the forest interior, the more cowbirds and the fewer songbirds as a result. Another example of a species benefiting from the proliferation of forest edge is the poison ivy.

In the case of developed lands juxtaposed to wild lands, problems with invasive exotics often result. Species such as Kudzu, Japanese Honeysuckle, and Multiflora Rose have done terrible damage to natural ecosystems.

Edge effects also apply to succession, that is where vegetation is spreading outwards rather than being encroached upon. Here different species will be more suited to the edges or central sections of the vegetation, resulting in a varied distribution. Edges themselves also vary with orientation - for example edges on the north or south will receive less or more sun than the opposite side (depending on hemisphere), resulting in differing vegetation patterns.

• Habitat fragmentation
• Landscape ecology

• Skole, D.L. and C. Tucker. 1994. Tropical deforestation and habitat loss fragmentation in the Amazon: satellite data from 1978-1988 (Science) 260:1905-1910